1. Field of the Invention
The present invention relates to a valve opening and closing timing control apparatus, and more particularly it relates to a valve opening and closing timing control apparatus which controls the opening and closing timing of engine valves linearly under a variety of engine conditions.
2. Description of the Related Art
There is a conventional technique which relates to the present invention and which is disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 3,111/1987, for example.
A conventional valve opening and closing timing control apparatus 80 will be hereinafter described with reference to FIG. 3. In the apparatus 80, there are provided a timing pulley 81 including a helical spline which is formed in its inner peripheral portion and a transmitter member 82 including a helical spline which is formed in its outer peripheral portion. Further, a ring-shaped piston means 83 including helical splines which are formed respectively in its inner and outer peripheral portions is disposed between the timing pulley 81 and the transmitter member 82, and it engages with them. Furthermore, an outer peripheral surface of the timing pulley 81 is engaged with a timing belt 91, and accordingly the timing pulley 81 is driven by a crank shaft (not shown) of an engine (not shown).
In the valve opening and closing timing control apparatus 80, the transmitter member 82 is fixed to a cam shaft 85 with a bolt 84 so that the transmitter member 82 and the cam shaft 84 do not rotate relatively to each other. The piston means 83 is divided into two component parts in its axial direction, and a spring 86 is disposed between the two component parts so as to construct what is called a scissors gear. With this construction, when rotary torques are transmitted from the timing pulley 81 to the transmitter member 82 by way of the piston means 83, it is possible to reduce the meshing noises resulting from the backlashes or the like between the helical splines.
In addition, a pressure receiving plate 87 is disposed on the left-hand side of the piston means 83 in FIG. 3, and it moves together with the piston means 83. Hence, the pressure receiving plate 87 moves the piston means 83 in the right and left directions in FIG. 3 depending on the hydraulic pressures in a hydraulic line 88. Further, a spring 90 is provided in a space 89 which is disposed on the right-hand side of the piston means 83 in FIG. 3, and it urges the piston means 83 to an initial position (i.e., to a left-most position in FIG. 3).
In the above-described conventional valve opening and closing timing control apparatus 80, however, the hydraulic pressures which are introduced into the hydraulic line 88 are controlled only by turning on and off a hydraulic control valve (not shown). As a result, only two occasions are available for the valve opening and closing timing. However, it is preferred that the valve opening and closing timing always be available at optimum and desired occasions, since an engine is required to have high performance.